1. Field of the Invention
This invention concerns an antiskid control device for use in a vehicle braking apparatus.
An antiskid device is generally adapted to perform braking operation, upon detecting that a skid state has appeared in vehicle wheel by virtue of the braking operation, while conducting the operation of eliminating the skid state by temporarily moderating the braking force and conducting the operation of recovering the braking force after the elimination of the skid state, and suitable control depending on the slip ratio of wheels is required for the control of the antiskid device.
That is, assuming the vehicle body velocity as V and the circumferential velocity of a wheel as Vw, the slip ratio S of the vehicle is defined as : ##EQU1## Further, assuming the friction coefficient between the wheel and the road surface in contact therewith and in the running direction as .mu., there is a relationship between the slip ratio S and the friction coefficient .mu. approximately as shown in FIG. 6.
In FIG. 6, curve a represents a relationship between the slip ratio S and the friction coefficient .mu. in a case where .mu. is high as on dried concrete road surface, curve b represents a relationship between the slip ratio S and the friction coefficient .mu. in a case where .mu. is as on snow-covered road surface and, further, curve c represents a relationship between the slip ratio S and the lateral friction coefficient .mu.L in the lateral direction of a wheel, that is, in the direction perpendicular to the running direction.
It can be seen from the figure that the friction coefficient .mu. is maximum near the slip ratio S32 0.2, which is decreased in the case where the wheels are locked, for example, by the rapid braking, the wheels assume a non-rotation state, and thereby the slip ratio S is increased, and the lateral friction coefficient .mu.L is rapidly lowered along with the increase of the slip ratio S.
Accordingly, in order to stop a vehicle within a minimum distance, it is desirable to apply braking force to wheels such that the friction coefficient .mu. during braking is always at the maximum value (0.2 in the example of FIG. 2). Further, in the case where the friction coefficient .mu. in the running direction is highest, since the lateral friction coefficient .mu.L also takes a relatively high value, thereby preventing the lateral slip of a vehicle body, such application of braking force provides a merit that the vehicle can be stopped quite safely.
2. Description of the Related Art
In view of the foregoing situations, there have been proposed mechanisms for automatically controlling the braking force so that the slip ratio S takes a favorable value. That is, there has been employed a method of setting a so-called simulated vehicle velocity by previously determining a vehicle velocity that is gradually reduced upon braking by way of experiment or simulation, and controlling the circumferential velocity of the wheels so as to obtain a slip ratio at a suitable value while referring to the characteristics of the simulated vehicle velocity.
However, the following problems are involved in the antiskid control device merely adapted to set an aimed wheel circumferential velocity in accordance with a simulated vehicle velocity and control the braking force to the wheels such that the actual circumferential velocity of the wheels agrees with the aimed wheel circumferential velocity.
That is, in a case where a simulated vehicle velocity with great deceleration is set and if the actual friction coefficient .mu. on the road surface is low, the wheel circumferential velocity is abruptly decreased to lock the wheels, by which the braking distance is increased and the wheels are liable to cause lateral slip. Further, in a case where a simulated vehicle velocity with small deceleration is set and if the actual friction coefficient .mu. on the road surface is high, the time for the non-braking state is unnecessarily extended to thereby increase the braking distance.
In view of the foregoing situations, the present inventors have proposed "antiskid control device" in Japanese Patent Application No. Sho 60-69285 (corresponding to U.S. Pat No. 4,671,579)
The device is adapted to measure the wheel circumferential velocities upon generation of a braking oil pressure reducing signal and upon re-pressurizing of the braking oil pressure, to determine the increase/decrease ratio for the wheel circumferential velocity, estimate the friction coefficient on the road surface based on the increase/decrease ratio, and open or close a valve for use in oil pressure control based on the estimated friction coefficient, and it has a feature capable of controlling the braking force properly in accordance with the change of the friction coefficient .mu. on the road surface.
That is, assuming the initial value of the vehicle velocity V as Vo, the initial value for the wheel acceleration/deceleration obtained by the differentiation of the detection data for the wheel circumferential velocity as Vro (where V is generally a negative value) and the time elapsed from a time of applying the braking force as t, the simulated vehicle velocity Vr is obtained by the calculation : EQU Vr=Vo+Vro.times.t (2)
Further, an aimed wheel circumferential velocity Vs is obtained from the simulated vehicle velocity Vr and the slip ratio S (generally 0.1-0.3) according to the calculation : EQU Vs=Vr.times.(1-S) (3)
and control is so performed that the circumferential velocity for each of the wheels coincides with the aimed circumferential velocity Vs of the wheels.
However, there have been found the following two disadvantages in the device described above :
The first disadvantage is that, since the friction coefficient .mu. on the road surface is set to each of the wheels, it is difficult to minimize the braking distance in a case where the wheels are respectively brought into contact with road surfaces of different friction coefficients .mu.. The second disadvantage is that the antiskid effect is exerted excessively under the effect of torsional moment acting on the wheel system upon braking operation on a road surface with high friction coefficient .mu. to thereby cause the undesirable phenomenon of increasing the braking distance.